In some switching systems (or controlled-switching systems), for example in "step-down" switching regulators such as that shown in FIG. 1, there is an electronic switch which requires a driver device having a supply terminal connected to a terminal of the switch.
As shown in FIG. 1, the switching system 100 comprises an N-channel MOS transistor (nMOS) 105 as an electronic switch. The transistor 105 is connected in the circuit shown by means of two terminals thereof, that is, its source and drain terminals, so as alternately to open and to close the connection between two branches of the circuit connected to these terminals. The transistor 105 is connected, by means of its drain, to a voltage supply indicated as the input voltage Vin (relative to ground). If this switch is opened and closed alternately, that is, if the transistor 105 is cut off and made conductive alternately, a series of voltage pulses of amplitude Vin, having a mean voltage value depending upon the duration of the intervals in which it is switched on and off is produced at the source of the transistor 105. A voltage Vout (relative to earth) is applied to a load 125 by means of a diode 110 and a filter constituted by an inductance 115 and a capacitor 120 (used to reduce the residual alternating component of the voltage at the source of the transistor 105). If the switching frequency of the transistor 105 is high enough, the load 125 is supplied with a direct-current voltage of a value equal to the mean value of the voltage at the source of the transistor 105.
The transistor 105 is controlled by a driving device (or driver 130) supplied by a suitable voltage supply produced, in the example shown, by means of a "bootstrap" circuit which uses a capacitor 135 connected by means of a diode 140 to a supply terminal Vcc. The supply terminals of the driver 130 which are connected to the terminals of the capacitor 135, are indicated Vhstrap and Vhsrc. The terminal Vhsrc is also connected to the source of the transistor 105.
The switching system 100 receives a logic input switching signal (Ldrive) which can assume two levels, one being a high or on level and the other a low or off level; the logic switching signal (Ldrive) is applied to the system 100 by means of a driver 145 supplied by a further voltage supply Vdd. For example, the switching-on signal is equal to the value Vdd of the supply voltage, and the switching-off signal is equal to a zero voltage (0V), that is, to ground potential level.
A translator circuit 150 is interposed between the driver 145 (supplied between Vdd and ground) and the driver 130 (supplied between Vhstrap and Vhsrc). This circuit 150 enables the logic switching signal Ldrive to be translated from a value between 0 and Vdd (for example 5V) to a value between Vhsrc and Vhstrap, where Vhstrap-Vhsrc is constant (for example 5V or 12V) but Vhsrc is variable with time. In the example shown in the drawing, Vhsrc varies from a value of 0V when the switch 105 is open to a value of Vin when the switch 105 is closed; it should be noted that, in certain cases, the voltage Vhsrc may even assume very high values of the order of some tens or hundreds of volts.
Some switching systems comprising various voltage translator devices known in the art are shown in FIGS. 2a-2c. With reference, in particular, to FIG. 2a, a simple voltage translator circuit which uses a Zener diode 205 connected between the supply terminal Vhstrap and the input of the driver 130 is shown. A resistor 210 which does not have a high value, is connected in parallel with the Zener diode 205 to prevent the switch from switching off too slowly. This translator circuit has a high consumption since, in the switching-on stage, the currents are quite high (of the order of a few hundreds of .mu.A up to a few mA).
To prevent this problem, as shown in FIG. 2b, it has been proposed to replace the resistor in parallel with the Zener diode 205 with a current generator 215 which is switched off during the switching-on stage. The switching-on current (Ion) and the switching-off current (Ioff) can thus be reduced to a few tens of .mu.A when switching has taken place without the logic state of the switch being lost. This circuit nevertheless also has a consumption in the switching-off stage since the switching-off current (Ioff) cannot be zero.
A different known solution which has a static consumption theoretically of zero and is used particularly when the voltage Vhsrc of the switch terminal reaches high values (tens or hundreds of V) is constituted by the circuit shown in FIG. 2c in which the driver signal is stored in a memory element 220 comprising, for example, a "latch" SR 225. The switching-on and switching-off currents (Ion and Ioff) in this case serve solely for switching the memory element 220 and are therefore pulsed currents with a duration of a few hundreds of ns up to 1 .mu.s. The signal is translated by means of current pulses generated as a result of a transition of the switching signal. In particular, a switching-on pulse is generated by the block 250 upon the transition of the switching signal from off to on whereas a switching-off pulse is generated by the block 255 upon the transition of the switching signal from on to off.
This translation circuit which is known in the art has some disadvantages. In order to ensure the correct state of the latch 225 after the start-up stage, the circuit described above in fact requires the latch 225 to be connected to a very high resistance 230 which can orient the latch 220 correctly during the rise of the supply voltage of the driver; in practice, however, this resistance 230 cannot have excessively high values, thus producing a current consumption which can bring about discharge of the bootstrap capacitance.
A further problem of the circuit of FIG. 2c is due to the fact that it cannot ensure congruence between the logic switching signals Ldrive and their translated values since the translation takes place by means of pulses generated in relation to a transition of the switching signal Ldrive and not in relation to its logic value. In fact, it is possible for a current pulse not to succeed in switching the memory element of the circuit, for example, by virtue of interference of short duration. In this case, the switch retains its previous state (open or closed) conflicting with the value of the switching signal Ldrive applied. For example, if the switching signal Ldrive changes from off to on, it generates a current switching-on pulse; if this pulse does not succeed in switching the switch, it will remain closed throughout the duration of the switching-on signal. The next transition of the switching signal from on to off will have no effect on the switch, simply confirming its already switched-off state. The switch will thus switch on only upon the subsequent transition of the switching signal from off to on. This problem is particularly serious in the case of a switching-off signal; if the switch is closed and the translator circuit does not succeed in storing the switching-off pulse, the switch remains closed with serious risks of damage to the components of the circuit.